Telomeres, the ends of eukaryotic chromosomes, are essential structures that mediate complete replication of chromosomal termini and cellular proliferation, and also insulate chromosome ends from degradation and end-to-end fusions. Studies in both yeast and mammalian cells have shown that the inability to replicate the telomere leads to telomere shortening and cellular senescence. Therefore, genes that ensure normal telomere function fall into the category of longevity assurance genes. The primary activity responsible for telomere replication is the enzyme telomerase; loss of this enzyme results in telomere shortening and an inability to proliferate. In contrast, the exact molecular mechanism by which protection of telomeres occurs is unclear. We have recently demonstrated that proteins that have been well characterized for their requirement in double strand break repair also play critical roles in telomere function. We have proposed that one of these protein complexes, the Ku heterodimer, is required to protect chromosomal termini from degradation; this activity is required in parallel with telomerase, as strains of yeast defective for both telomerase and Ku exhibit a greatly accelerated senescence phenotype. The second DNA repair complex, the Mre11/Rad5O/Xrs2 complex, plays a different role at the telomere, in that it is required for the telomerase-mediated pathway for telomere replication, rather than for end protection. These results pose a fundamental question regarding how recruitment of Ku and Mre11/Rad50/Xrs2 to both random double strand breaks and the telomere can have such different consequences in vivo: if the same proteins that mediate repair of DNA strand breaks are also present at the telomere, what prevents telomeres from similarly undergoing end-to-end fusions? We hypothesize that these repair proteins have activity(s) at the telomere that are distinct from their action at double strand breaks. We propose to address this hypothesis via the identification of mutants in these DNA repair genes that uncouple telomere function from double- strand break repair. Second, we present molecular and biochemical assays designed to characterize the roles of Ku and Mre11/Rad50/Xrs2 at the telomere. Finally, we propose to use several specific genetic screens to identify and characterize new genes that function with Ku and the Mre11/Rad5O/Xrs2 complex at the telomere.